Kinetic investigations of the electrical behavior and enzymatic partial reactions of the ATP-dependent Na+,K+ and Ca 2+ pumps have demonstrated that the phosphoenzyme conformational transition is an electrogenic in the transport cycle. Studies of the mechanism of Na+ -H+ exchange in renal brush border membranes have established that extravesicular protons inhibit Na+ uptake by competing for a common binding (transport) site on the carrier protein. Single channel investigations of chloride channels from skeletal muscle sarcoplasmic reticulum have demonstrated that ATP and cyclic AMP activate the channel by stimulating the formation of an open state intermediate with a prolonged open time. The mechanism of activation may involve phosphorylation of glycogen phosphorylase which is bound to the SR membrane as an extrinsic protein. Direct visualization of plasma membrane domains in human skin fibroblasts by fluorescence microscopy has been accomplished by selective labelling of the cell membrane with fluorescent phospholipids. Comparison of the labelling patterns in fibroblasts from young and old patients indicate that the domains enlarge during aging secondary to loss of the barriers that surround them.